This invention relates to apparatus for separating solid particles from liquids.
Separation of solids from liquids is a problem faced in a diverse range of processing situations. In the past it has been known to discharge unprocessed or partly processed liquid waste into local waterways. This has led to devastating pollution and the death of many waterways in industrialised countries. In recent years there has been increased focus on attempts to develop adequate filtration systems that can separate harmful solids from waste liquids.
The removal of phosphates and suspended solids from sewerage waste water is one example of a situation requiring improved filtration. Another problem requiring attention is the separation of particles from car wash discharge water.
It is known to use porous membranes for filtration purposes. The liquid containing solid particles passes through the membrane and the particles are trapped by the pores. A wide variety of membrane types are available on the market. They vary by material as well as by pore size. Membranes may be woven or non-woven metallic thread, ceramic, plastic, cloth or a hybrid of two or more materials. Whatever the membrane material, the operation of the membrane can be characterised by a number of parameters, including construction, performance (eg cross-flow or in-depth filtration), pore size and porosity.
For efficient operation, the pore size of the membrane filter must be less than the size of the smallest particles in the liquid. If this criteria is met the liquid will flow through the membrane but the particles will be captured. This leads to the main disadvantage of this type of filter, namely clogging of the pores. After a period of time the trapped solids will block the pores of the membrane and prevent or restrict the flow of liquid. This is particularly a problem for membranes having small pore size of, say, less than 5 xcexcm.
Once the flow of liquid through the filter is reduced below acceptable levels it is necessary to clean the filter. Typically this is achieved by back flushing to wash the blocking material from the pores. Obviously, normal filtration operations must be suspended during the cleaning operation. The disruption is minimised if the back flushing is a simple reversal of the filtration operation but more often back flushing involves a different process. Many back flush operations require steam or compressed air to be directed back through the filter. This leads to a complex system for performing all the required operations.
Even with a sophisticated system it is often the case that back flushing will not return the filter to the same flow rates achieved with a clean filter. Repeated back flushing degrades the filtration membrane which must be replaced regularly.
One example of a filtration application having stringent requirements is the separation of blood plasma from whole blood. An apparatus for this application is described in U.S. Pat. No. 4,755,300 assigned to Haemonetics Corporation. This patent describes an apparatus in which an elongated core element is rotated within an elongate hollow container. A membrane is disposed over the exterior surface of the core element or interior surface of the container. Rotation of the core element in the container produces laminar flow of the blood in the gap between the core element and the container. The resultant shearing action produces a laminar boundary layer immediately adjacent the membrane that consists of plasma only while the suspended red blood cells are repelled towards the centre of the gap. This allows the plasma to be collected.
The requirement for laminar flow in the Haemonetics Corporation device places severe limits on its usefulness. While useful for separating plasma from blood, the apparatus does not have broader application.
One apparatus for overcoming some of the deficiencies of prior art filtration systems is described in Australian Patent Application number 50399/96 in the name of Kevin Douglas McGrath. In the McGrath filtration apparatus feed liquid flows across a membrane from an inlet port to an outlet port. Filtrate is drawn through the membrane to a filtrate outlet port. Because the feed liquid flows across the surface of the membrane the amount of material trapped permanently in the pores of the membrane is reduced.
The inventor has found that an apparatus having a greater filtrate specific flow rate than the above apparatus is desirable. Furthermore, some degree of back flushing is still required in the prior art system. It is desirable to minimise or eliminate the need for back flushing.
It is an object of the present invention to provide a filtration apparatus having a relatively higher specific flow rate compared to the known prior art apparatus.
It is a further object of the invention to provide a useful alternative to known filtration apparatus.
Further objects will be evident from the following description.
In one form, although it need not be the only or indeed the broadest form, the invention resides in a filtration apparatus comprising:
a hollow body defining a chamber;
a filter element concentrically housed in the chamber, said filter element consisting of a support and a membrane attached to said support, and having a cross flow side and a filtrate side;
a drive mechanism coupled to said filter element and causing rotation thereof;
at least one inlet port communicating with the chamber for ingress of suspension to be filtered and at least one outlet port communicating with the chamber for egress of concentrate;
a flow path from the inlet port across the cross flow side of the filter element to the outlet port such that filtrate from the suspension passes through the filter element to the filtrate side and remaining concentrate passes out the outlet port; and
a filtrate port communicating with the filtrate side of the filter element for removal of filtrate from the apparatus.
In preference, the support may be conical, cylindrical or a combination of both. The support has a plurality of apertures that allow passage of filtrate from the membrane to a channel leading to the filtrate outlet port.
The membrane is suitably formed as a composite structure having a number of layers. In the preferred structure the pore size of each layer increases from the inlet cross flow side of the membrane to the outlet filtrate side.
In preference, there are multiple inlet ports and multiple outlet ports.
A suitable drive mechanism consists of a spindle on which the cylindrical support is mounted, a motor and a coupling between the motor and the spindle. The coupling may conveniently be a belt and pulley system.
For collection of the filtrate there may be a central channel formed along a portion of the axis of the spindle. The apertures in the cylindrical support communicate with an upper end of the channel and the filtrate port communicates with a lower end. A rotary coupling may be used between the filtrate port and the channel.
The flow path preferably promotes turbulent flow in the chamber. To further promote turbulent flow the filtration apparatus may further comprise a plurality of vanes projecting into the chamber from the hollow body. The vanes are most suitably elongate projections aligned parallel to an axis of the cylindrical support.
The filtration apparatus may further comprise cleaning means for enhancing cleaning of the membrane by a pulse pressure drop in the chamber that causes an instantaneous reverse flow through the membrane. The cleaning means may comprises a pneumatic vibrator in pressure communication with the chamber for applying the reduced pressure pulse to the chamber. The pneumatic vibrator may optionally vibrate to vary the pressure drop during the reverse flow thereby enhancing the cleaning.